As a rotary inertia mass damper, there has been known a rotary inertia mass damper disclosed in Patent Literature 1. The rotary inertia mass damper includes a screw shaft, a nut member, an outer barrel, and an additional weight. The screw shaft has a spiral male thread, and one end thereof is fixed to a construction. The nut member is threadedly engaged with the screw shaft. The outer barrel supports the nut member so as to be freely rotatable, and is fixed to the construction. Rotation is applied to the additional weight by the nut member.
When relative vibration generated in the construction due to an earthquake or the like is input between the screw shaft and the outer barrel, axial acceleration is caused in the screw shaft due to the vibration, and the axial acceleration is converted into angular acceleration of the nut member threadedly engaged with the screw shaft. The angular acceleration is transmitted, to the additional weight mounted to the nut member so that the nut member and the additional weight are rotated. The nut member and the additional weight integrally construct a rotary body, and a rotational torque generated in the rotary body is expressed by a product of an inertial moment of the rotary body and the angular acceleration. The rotational torque is reversely converted by the nut member and the screw shaft each time the axial acceleration of the screw shaft is reversed, and is applied to the screw shaft as an axial reaction force.
As described above, when a ball screw device in which the nut member is threadedly engaged with the screw shaft is used, and the vibration applied to the construction is converted into a rotary motion of the rotary body, the axial acceleration of the screw shaft is amplified with an amplification factor in accordance with a lead of the male thread of the screw shaft when the axial acceleration of the screw shaft is converted into the angular acceleration of the nut member. Further, also when the rotational torque generated in the rotary body is reversely converted into the axial reaction force of the screw shaft, the rotational torque is amplified with an amplification factor in accordance with the lead of the male thread. Therefore, in the rotary inertia mass damper, even when the mass of the rotary body is small, a large reaction force can be applied to the screw shaft. Thus, a significant vibration damping effect can be obtained while attaining downsizing of the device.
Meanwhile, the rotational torque generated in the rotary body is expressed, by the product of the inertial moment of the rotary body and the angular acceleration. Thus, when excessive axial acceleration is input from the construction to the screw shaft, and is amplified so that excessive angular acceleration is generated in the nut member, the rotational torque generated in the rotary body also becomes excessive. Therefore, when the rotational torque is reversely converted into the axial reaction force of the screw shaft as it is, there is a fear in that the screw shaft or the nut member may be broken. Further, even though the screw shaft and the nut member are not broken, when the excessive axial reaction force is applied from the rotary inertia mass damper to the construction, there is also a fear in that the construction may be broken by the axial reaction force.
In this regard, in the rotary inertia mass damper disclosed in Patent Literature 1, friction members are provided between the nut member and the additional weight, and an upper limit value is set for the rotational torque transmitted between the nut member and the additional weight. Therefore, when the rotational torque transmitted between the nut member and the additional weight exceeds the upper limit value set for the friction members, the additional weight, is separated from the nut member, and the inertial moment of the rotary body is generated only by the nut member. With this, the rotational torque reversely converted into the axial reaction force of the screw shaft is suppressed, thereby preventing the excessive axial reaction force from being applied to the screw shaft.